Disk shaped components are generally subjected to two major types of loading during operation. The first loading type is the centrifugal force due to disk rotation. The second loading type arises from temperature gradient in the radial direction of the disk. The relative magnitudes of these two types of loading depend upon the rotation speed, density of the disk material and the severity of the temperature gradient. In general, the centrifugal load tends to generate more stress than the thermal load, but the thermal load could also have a significant impact upon the disk's mechanical integrity.
Referring now to FIGS. 1 and 2, for purposes of example, the disk shaped component may be a turbine disk commonly used in various engines. In FIG. 1, a turbine disk 10 is enclosed within a turbine support casing 22 of a gas turbine engine 20. The turbine disk 10 includes rotor blades 24 mounted thereto and disposed within a hot combustion gas stream 18 between two vanes 26, 28. The turbine disk 10 is normally cooled by compressor bleed air 12 flowing from the disk bore 14 to the disk rim 16, where the air is discharged into a hot gas stream 18. With the rim 16 being heated by the hot combustion gas stream 18, the temperature gradient in the disk 10 is such that the rim 16 is hotter than the disk bore 14. The hotter rim 16 would expand more than the relatively cooler bore 14 assuming the same coefficient of thermal expansion (CTE) for the two regions. The mismatch in thermal growth generates tensile thermal stress at the bore 14 and compressive stress at the rim 16. Since the tensile stress due to centrifugal force tends to peak at the disk bore 14, the combined thermal and mechanical stress reaches a maximum also at the bore 14.
Under both loading conditions, peak tensile stresses occur at the disk center or disk bore and therefore reinforce each other, creating the worst stress situation in the disk. For the bored disk, the peak stress is twice that for a solid disk. Although it would be ideal to always utilize a solid disk, it is often necessary to have a bore in a disk in order to pass cooling air or integrate a shaft of a front drive from, e.g., generator, propeller, etc., as illustrated in FIG. 1. These circumstances sometimes make it difficult to meet the requisite disk mechanical integrity requirements, especially for materials of relatively low strength.
Therefore, there exists a need for lowering the thermal stress of a bored disk in order to improve the overall mechanical integrity.